A wide variety of semiconductor devices, and methods of making semiconductor devices, are known. Some of these devices are designed to emit light, such as visible or near-visible (e.g. ultraviolet or near infrared) light. Examples include light emitting diodes (LEDs) and laser diodes. Another example is a stack of semiconductor layers that forms a re-emitting semiconductor construction (RSC).
Unlike an LED, an RSC does not require an electrical drive current from an external electronic circuit in order to emit light. Instead, the RSC generates electron-hole pairs by absorption of light at a first wavelength λ1 in an active region of the RSC. These electrons and holes then recombine in potential wells in the active region to emit light at a second wavelength λ2 different from the first wavelength λ1, and optionally at still other wavelengths λ3, λ4, and so forth depending on the number of potential wells and their design features. The initiating radiation or “pump light” at the first wavelength λ1 is typically provided by a blue, violet, or ultraviolet emitting LED coupled to the RSC. Exemplary RSC devices, methods of their construction, and related devices and methods can be found in, e.g., U.S. Pat. No. 7,402,831 (Miller et al.), U.S. Patent Application Publications US 2007/0284565 (Leatherdale et al.) and US 2007/0290190 (Haase et al.), PCT Publication WO 2009/048704 (Kelley et al.), and pending U.S. application Ser. No. 61/075,918, “Semiconductor Light Converting Construction”, filed Jun. 26, 2008, all of which are incorporated herein by reference.
FIG. 1 shows an illustrative device 100 that combines an RSC 108 and an LED 102. The LED has a stack of LED semiconductor layers 104, sometimes referred to as epilayers, on an LED substrate 106. The layers 104 may include p- and n-type junction layers, light emitting layers (typically containing quantum wells), buffer layers, and superstrate layers. The layers 104 may be attached to the LED substrate 106 via an optional bonding layer 116. The LED has an upper surface 112 and a lower surface, and the upper surface is textured to increase extraction of light from the LED compared to the case where the upper surface is flat. Electrodes 118, 120 may be provided on these upper and lower surfaces, as shown. When connected to a suitable power source through these electrodes, the LED emits light at a first wavelength λ1, which may correspond to blue or ultraviolet (UV) light. Some of this LED light enters the RSC 108 and is absorbed there.
The RSC 108 is attached to the upper surface 112 of the LED via a bonding layer 110. The RSC has upper and lower surfaces 122, 124, with pump light from the LED entering through the lower surface 124. The RSC also includes a quantum well structure 114 engineered so that the band gap in portions of the structure is selected so that at least some of the pump light emitted by the LED 102 is absorbed. The charge carriers generated by absorption of the pump light move into other portions of the structure having a smaller band gap, the quantum well layers, where the carriers recombine and generate light at the longer wavelength. This is depicted in FIG. 1 by the re-emitted light at the second wavelength λ2 originating from within the RSC 108 and exiting the RSC to provide output light.
FIG. 2 shows an illustrative semiconductor layer stack 210 comprising an RSC. The stack was grown using molecular beam epitaxy (MBE) on an indium phosphide (InP) wafer. A GaInAs buffer layer was first grown by MBE on the InP substrate to prepare the surface for II-VI growth. The wafer was then moved through an ultra-high vacuum transfer system to another MBE chamber for growth of II-VI epitaxial layers used in the RSC. Details of the as-grown RSC are shown in FIG. 2 and summarized in Table 1. The table lists the thickness, material composition, band gap, and layer description for the different layers associated with the RSC. The RSC included eight CdZnSe quantum wells 230, each having a transition energy of 2.15 eV. Each quantum well 230 was sandwiched between CdMgZnSe absorber layers 232 having a band gap energy of 2.48 eV that could absorb blue light emitted by an LED. The RSC also included various window, buffer, and grading layers.
TABLE 1Ref-erenceThicknessBand Gap/No.Material(nm)Transition (eV)Comment230Cd0.48Zn0.52Se3.12.15quantum well232Cd0.38Mg0.21Zn0.41Se82.48absorber234Cd0.38Mg0.21Zn0.41Se:Cl922.48absorber236Cd0.22Mg0.45Zn0.33Se1002.93window238Cd0.22Mg0.45Zn0.33Se →2502.93-2.48gradingCd0.38Mg0.21Zn0.41Se240Cd0.38Mg0.21Zn0.41Se:Cl462.48absorber242Cd0.38Mg0.21Zn0.41Se → 2502.48-2.93gradingCd0.22Mg0.45Zn0.33Se244Cd0.39Zn0.61Se4.42.24II-VI buffer246Ga0.47In0.53As1900.77III-V bufferFurther details of this and other RSC devices can be found in PCT Publication WO 2009/048704 (Kelley et al.).
Since the layers of the RSC are composed of semiconductor materials, and semiconductor materials typically have relatively high refractive indices, light at the second wavelength λ2, and any other light generated within the RSC, is susceptible to becoming trapped inside the RSC by total internal reflection at outer surfaces thereof, and to being absorbed within the RSC instead of contributing to the emitted output. Therefore, to reduce this loss mechanism, surface texturing of one or more surfaces of the RSC to enhance extraction efficiency of long wavelength light generated within the RSC has been proposed. See e.g. PCT Publication WO 2009/048704 (Kelley et al.).